European Patent Application No. 0 422 870 A2 discloses a drop on demand ink jet printing apparatus that includes a piezoelectric substrate defining an array of parallel uniformly spaced channels provided with respective droplet ejection nozzles, an ink supply common to the channels, and electrically actuable means for applying pulses of energy to ink in the channels to effect droplet ejection from the channels. The length of the channels is chosen to provide a high longitudinal resonant frequency, and the pulses of energy are applied at or near the resonant frequency of the channels to effect droplet ejection. The channel densities in such print heads can be from 2 to 16 per millimeter.
At such high channel densities, bubbles which form in the ink may block the entrance to a channel, thereby preventing operation of the ink jet.
U.S. Pat. No. 4,466,005 discusses the problem of removing air bubbles from a drop on demand ink jet print head and solves the problem by applying a plurality of drive signals of different pre-selected voltages and frequencies in predetermined cycles to remove air bubbles from the print head. This approach is not effective however when the air bubbles are too large and the surface tension of the ink prevents the air bubbles from entering the ends of the channels in the high density print heads of the type disclosed in EP 0 422 870 A2.